This invention relates to a method and system for determining the group delay of a multimode, dispersive acoustic guided wave, and more particularly to systems and methods which employ the group delays to determine thickness, locate discontinuities, measure distances, and locate the source in multimode dispersive mediums.
The merits of Lamb wave testing for the nondestructive evaluation (NDE) of thin plates and pipes are well known. When a large area of a structure is to be inspected, Lamb waves are more attractive than bulk waves since they can propagate over long distances to inspect a wider area. However, it is a great misfortune that Lamb waves are multimode by nature. That is, there exist at least two wave modes at any given frequency, thus making the signals complicated and difficult to interpret. Further they are dispersive, meaning that for any given mode the velocity of the waves varies with frequency.
In general, there are two approaches taken to address this unfavorable circumstance. The first coaxes a single mode into dominance. The other accepts the multimode nature of the signal and treats it at a single analysis level.
However, this latter approach has not lent itself to determining distances so that the locations of a flaw or boundary, the location of an unknown acoustic source, or the distance between positions could be determined in a multimode dispersive medium.
It is therefore an object of this invention to provide an improved method and system for interpreting multimode dispersive signals.
It is a further object of this invention to provide such an improved method and system which uses time-frequency analysis or spectrotemporal analysis to interpret multimode dispersive signals.
It is a further object of this invention to provide such an improved method and system which can determine flaw location, source location, distances and even thickness of multimode dispersive media.
It is a further object of this invention to provided such an improved method and system which calculates group delays in multimode dispersive media.
The invention results from the realization that a simple and effective determination of group delays in multimode dispersive media can be achieved by frequency decomposing multimode dispersive acoustic guided waves and selecting a frequency which identifies the group delays for each mode occurring at that frequency and using that group delays in conjunction with either the group velocities or distance to obtain the other to locate the source of the waves, flaw locations, distances and thickness.
This invention features a method of locating discontinuities in a multimode dispersive medium including exciting at a first position, a multimode dispersive acoustic guided wave in a medium and sensing a second position, both a direct arrival and the reflected multimode dispersive acoustic guided wave. The multimode dispersive acoustic guided wave is frequency decomposed using a spectro-temporal analysis and then a frequency is selected which identifies the group delay, tg, of the direct and reflected guided waves for each mode occurring at that frequency. A mode is selected and a determination is made of the time difference, xcex94tg, between the direct and reflected signals from the group delays of the selected mode. The group velocity cg is determined from the dispersion curves and the distance L between the discontinuity and the second position is computed according to the expression L=cgxc2x7xcex94tg/2.
The invention also features a method of distance measurement between positions on a multimode dispersive medium including exciting at a first position a multimode dispersive acoustic guided wave in the medium and sensing at a second position the multimode dispersive acoustic guided wave. The multimode dispersive acoustic guided wave is frequency decomposed using spectral temporal analysis. A frequency is selected which identifies the group delay tg of the guided wave for each mode occurring at that frequency. A mode is selected and a determination is made of the group delays tg from the first position. The group velocity cg is determined from the dispersion curve and the distance L between the two positions is computed from the group delay tg and group velocity cg.
This invention also relates to a method of locating a source in a multimode dispersive medium including sensing at each of a plurality of positions a different multimode dispersive acoustic guided wave emanating from a source and frequency decomposing each of the multimode dispersive acoustic guided waves using spectrotemporal analysis. A frequency is selected which identifies the group delay tg of each guided wave for each mode occurring at that frequency. A mode is selected and a determination is made of the group delays tgn of the selected mode for each guided wave. The group velocity cgn is determined from the dispersion curves for each guided wave. The difference between pairs of group delays is calculated and they are multiplied by their respective group velocities to obtain the differences between pairs or distances of the positions from the source. The distance differences are combined to obtain the location of the source.
This invention also features a method of thickness measurement in a multimode dispersive medium including exciting in a first position a multimode dispersive acoustic guided wave in the medium and sensing at a second position the multimode dispersive acoustic guided wave. The multimode dispersive acoustic guided wave is frequency decomposed using spectro-temporal analysis. A frequency is selected which identifies the group delay tg of the guided wave for each mode occurring at that frequency. A mode is selected and a determination is made of the group delays from the first position. The distance L is identified between the first and the second positions. A mode is selected and a determination is made of the group delay tg from the first position. The group velocity cg is calculated in accordance with cg      c    g    =      L          t      g      
and a determination is made of the thickness from the dispersion relationship.
In a preferred embodiment the sensing occurs at a plurality of second positions surrounding the first position.
This invention also relates to a system for locating discontinuities in a multimode dispersive medium comprising means for exciting at a first position a multimode dispersive acoustic guided wave in a medium and means for sensing at a second position both the direct and reflected multimode dispersive acoustic guided wave. There are means for frequency decomposing the multimode dispersive acoustic guided wave using spectral temporal analysis and means for selecting a frequency which identifies the group delay tg of the direct and reflective guided wave for each mode occurring at that frequency. There are means for selecting a mode and determining the time difference xcex94tg between the direct and reflected signals from the group delays of the selected mode. There are means for determining the group velocity cg from the dispersion curves and means for computing the distance L between the discontinuity and the second position in accordance with the expression L=cgxc2x7xcex94tg/2.
This invention also features a system for distance measurement between positions on a multimode dispersive medium including means for exciting at a first position a multimode dispersive acoustic guided wave in the medium and means for sensing at a second position the multimode dispersive acoustic guided wave. There are means for frequency decomposing the multimode dispersive acoustic guided wave using spectral temporal analysis and means for selecting a frequency which identifies the group delay tg of the guided wave for each mode occurring at that frequency. There are means for selecting a mode and determining the group delays tg from the first position. There are also means for determining the group velocity cg for the dispersion curves and means for computing the distance L between the two positions from the group delay tg and group velocity cg in accordance with the expression L=tgxc2x7cg.
This invention also features a system for locating a source in a multimode dispersive medium including means for sensing each of a plurality of positions a different multimode dispersive acoustic guided wave emanating from a source and means for frequency decomposing each of the multimode dispersive acoustic guided waves using spectral temporal analysis. There are means for selecting a frequency which identifies the group delay tg of each guided wave for each mode occurring at that frequency and means for selecting a mode in determining the group delays tgn of the selected mode for each guided wave. There are means for determining the group velocity tgn for the dispersion curves for each guided wave and means for calculating the difference between pairs of group delays and multiplying them by their respective group velocity to obtain the differences between pairs of distances of the positions of the source. There are also means for combining the distance differences to obtain the location of the source.
This invention also features to a system for thickness measurement in a multimode dispersive medium including means for exciting in a first position a multimode dispersive acoustic guided wave in the medium and means for sensing in a second position the multimode dispersive acoustic guided wave. There are means for frequency decomposing the multimode dispersive acoustic guided wave using spectral temporal analysis and means for selecting a frequency which identifies the group delay tg of the guided wave for each mode occurring at that frequency. There are means for selecting a mode and determining the group delays from the first position. There are means for identifying the distance L between the first and second positions, means for selecting a mode and determining the group delay tg for the first position, and means for calculating the group velocity cg in accordance with the expression cg=L/tg, and there are means for determining thickness from the dispersion relationship.
In a preferred embodiment, the means for sensing includes sensor means at a plurality of positions surrounding the first position.